A classical implementation of switching between two signals V1 and V2 could be a direct switching with a logical control as shown in circuit 100 of FIG. 1. Two CMOS switches S1 and S2 are used and they are controlled by a reversed logical control signal (CTRL and inverted CTRL through inverter 102). Either signal V1 or V2 is passed through operational amplifier 104 to provide the VOUT output signal. If the two signals have different signal voltages, there will be a jump or step at the output point 106. In some applications, this jump or step of the output signal could create a problem. For example, in an audio application, if the output signal goes to a speaker, the jump will create an undesirable pop noise at the speaker. In this case, a smooth transition between the two input signals is obviously preferred.
A traditional digital solution 200 for a smooth transition between switched input signals uses resistor networks 212 and 214, as well as corresponding switching networks 216 and 218 to divide a big step into small ones as shown in FIG. 2. Circuit 200 includes an input for receiving a first input signal V1, and a second input for receiving a second input signal V2. Circuit 200 also includes an output operational amplifier or buffer 204, soft-switching clock generator 206, a switch counter 208, and a logic controller 210. Circuit 200 also typically includes a final post processing low pass filter 220. By increasing the number of small steps and the soft-switching time, the pop noise is reduced. In the solution shown in FIG. 2, the resistors and parasitic components will deteriorate performance due to increased noise and Total Harmonic Distortion (THD). For some configurations of step numbers and soft-switching times, undesired audio tones are generated. A low pass filter is needed to remove high frequency digital spikes associated with circuit 200. The digital circuit 200 shown in FIG. 2 presents difficult trade-offs between pop noise and other performance considerations.
What is desired, therefore, is a simple analog switching circuit for providing a smooth transition between two input signals, and without the need for any post-processing or filtering, or without the use of complex digital circuitry.